Note:The United States enters World War II after the attack at Pearl Harbor, a global war that involved the vast majority of the world's nations and including all of the great powers - http://en.wikipedia.org/wiki/World_War_II

Note:The Korean War was a war between the Republic of Korea (South Korea), supported by the United Nations, and the Democratic People's Republic of Korea (North Korea), at one time supported by the People's Republic of China and the Soviet Union - http://en.wikipedia.org/wiki/Korean_War

Note:The Vietnam War was a long, costly armed conflict that pitted the communist regime of North Vietnam and its southern allies, known as the Viet Cong, against South Vietnam and its principal ally, the United States. The divisive war, increasingly unpopular at home, ended with the withdrawal of U.S. forces in 1973 and the unification of Vietnam under Communist control two years later. More than 3 million people, including 58,000 Americans, were killed in the conflict. - http://en.wikipedia.org/wiki/Vietnam_War